He says harsh interrogation methods may have yielded useful information but that they should still be ruled out. His stance suggests complications in ending Bush-era counter-terrorism tactics.

WASHINGTON — In a strikingly defensive explanation of his stance on Bush-era anti-terrorism tactics, President Obama on Wednesday acknowledged for the first time that the harsh interrogation techniques he has banned might have yielded useful information, but that he was nonetheless willing to rule them out on moral grounds.

It was a nuanced performance as Obama walked viewers of his prime-time news conference through a policy that has led him to declare tactics such as waterboarding torture but to stop short of advocating prosecution of the architects of the practices.

He conceded that "it may be harder" to get information, but what "makes us, I think, still a beacon to the world is that we are willing to hold true to our ideals, even when it's hard, not just when it's easy."

Coming as Obama confidently assessed his work during his first 100 days in office, his comments on torture underscored a gnawing dilemma: His desire to roll back elements of President Bush's "war on terrorism" could be more complicated than he had envisioned.

Obama is caught between growing public sentiment against igniting a national debate over past interrogation tactics and a still-emerging insistence by his liberal base that is growing more aggressive in its calls for investigations and prosecutions.

In conceding that intelligence was gleaned from the harsh techniques, Obama may be making himself vulnerable to arguments by former Vice President Dick Cheney and other conservatives that he is making the country less secure.

At least one survey this week showed that Cheney's arguments have support and that the shadow of Sept. 11 looms larger in the public mind than Obama might have initially realized.

A majority of Americans, 53%, said they opposed Obama's release of classified memos detailing the Bush administration's legal rationale for the interrogation tactics used on suspected terrorists, while just 40% supported it, according to an NBC News/Wall Street Journal survey.

The poll also found that half of Americans opposed Obama's order to close the Guantanamo Bay prison for terrorism suspects, and that a clear majority opposed the idea of criminal investigations of Bush administration figures.

Moreover, even though a majority believes that America used torture, a plurality said that the interrogations "helped by extracting valuable information."

Obama's remarks showed that he believes he must take on the former vice president in what is emerging as a battle to either justify or discredit harsh interrogation techniques.

The issue is not going away, now that the Pentagon has agreed to release a series of photos of abused inmates in Iraq and Afghanistan, the result of a lawsuit brought by the American Civil Liberties Union.

Obama revealed Wednesday that he had read classified memos recently cited by Cheney as evidence that the CIA's interrogation program produced information that prevented terrorist attacks.

Cheney has formally requested the declassification of those documents, and U.S. intelligence officials have indicated that the memos, or at least portions of them, are likely to be released.

Obama did not dispute Cheney's assertions about the memos but appeared to try to blunt their potential impact by shifting the argument.

The assertion that the CIA's methods worked doesn't answer what Obama called the core question: "Could we have gotten that same information without resorting to these techniques?" Obama asked. "And it doesn't answer the broader question: Are we safer as a consequence of having used these techniques?"

Obama was asked, twice, whether the Bush administration engaged in torture, which would be illegal. He tried to draw a difficult distinction, never going so far as to say that his predecessor's policies broke the law.

"I believe that waterboarding was torture," Obama said. "And I think that whatever legal rationales were used, it was a mistake."

Obama harked back to the restraint shown by Prime Minister Winston Churchill when Britain was under German attack in World War II.

"London was being bombed to smithereens [and] had 200 or so detainees. And Churchill said, 'We don't torture,' " Obama said. "Churchill understood, you start taking shortcuts, and over time, that corrodes what's best in a people."

To many human rights advocates calling for the appointment of a special prosecutor or a public inquiry, Obama's answers were remarkable for what he did not say: that he would back an investigation. So activists tried to put those words into his mouth later Wednesday.

Amrit Singh, a lawyer for the ACLU, said that Obama's statements led to the "inevitable conclusion that government officials who authorized torture must be held accountable for violating the law." Amnesty International issued a stern statement declaring that Obama's comment on waterboarding "means criminal investigations must follow."

But, to the disappointment of his liberal base, Obama clearly did not want to go there.

"It's a grave disappointment," said Michael Ratner, president of the Center for Constitutional Rights.

"He admits what was done was torture. And yet he won't dot the 'i' or cross the 't.' Why not?"

Ratner answered his own question, pointing to conservatives' arguments that Obama's actions are putting the country at risk.

"It's having an effect," Ratner said. "It's out there, and this argument is up for grabs, which is why Obama's going to have to be firmer as we move forward."